For years, drug companies have known that their days of plenty were numbered, that the moment would arrive when the best-selling drugs that had driven two decades’ worth of profits would lose their patent protection and succumb to competition from generic alternatives. Without new blockbusters to replace them, profits would tumble. For Pfizer, that day has arrived. Pfizer profited from hits like Lipitor and Viagra, and swallowed up smaller companies from the 1990s onward. But it has no immediate successor to Lipitor, the best-selling drug in history, which lost patent protection last fall. The problem was punctuated on Tuesday when the company said that profit declined 19 percent last quarter, largely because of declines in Lipitor sales.